1. Field of the Invention
This invention relates broadly to mechanisms for generating electricity. More particularly, this invention relates to mechanisms for generating electricity using piezoelectric materials.
2. State of the Art
Piezoelectricity is the result of charge displacement within a crystalline structure which lacks a central symmetry. Piezoelectric elements when subjected to a mechanical load (e.g., vibration, compression, and/or flexing) induce an electrical charge on opposite faces of a piezoelectric material. In prior art piezoelectric elements have been used for actuators, transducers, resonators, transformers, micro generators, and sensors of all types. Recently piezoelectric elements have been researched and developed for energy scavenging. The piezoelectric element functions as a capacitor in response to stress or strain.
When a piezoelectric material is subjected to a compressive or tensile stress, an electric field is generated across the material, creating a voltage gradient and a subsequent current flow due to compressive or tensile stress which seeks equilibrium. The current flow is provided by a conductive material that allows the unequal charge of the piezoelectric material to equalize by moving the unequal charge off from the piezoelectric material. Piezoelectric materials generate high voltage and low current electricity. The piezoelectric effect is reversible in that piezoelectric material, when subjected to an externally applied voltage, can change shape. Direct piezoelectricity of some substances (e.g., quartz, Rochelle salt) can generate voltage potentials of thousands of volts.
Piezoelectric materials store energy in two forms, as an electrical field, and as a mechanical displacement (strain). The relationship between strain and the electric field is given by SC=1/ST(SR−(d*e)) where “SC” is the compliance of the piezoelectric element in a constant electric field, “SR” is the mechanical deformation and “d” is the piezoelectric charge constant. The charge produced when a pressure is applied is: Q=d*P*A, where P is the pressure applied and A is the area on which the pressure is applied. Utilizing multiple piezoelectric stacks on top of one another and connecting them in parallel increases the charge in relationship to pressure. The output voltage generated can be expressed as the total charge of the stack divided by the capacitance of the stack.
In the prior art, piezoelectric materials have been used to scavenge energy from vibration energy induced by wind, ocean waves, ambient sound, automobile traffic, the deformation of an automobile tire, and the foot strike of a human being on a floor. However, the prior art methodologies have resulted in very low power output, which makes such solutions suitable only for low power applications.
Thus, there remains a need in the art for systems and methodologies that generate electricity by applying pressure gradients to piezoelectric material in manner that is suitable for a wide range of power supply applications, such as residential or commercial power supply applications.